An image forming apparatus has an electric load such as a transfer roller, and a power supply device that controls power supply to the electric load. A known power supply device includes a PWM circuit that outputs a PWM (pulse width modulation) signal and a high voltage generation device that supplies voltage that depends on a pulse width of the PWM signal to the electric load. The PWM circuit detects an instantaneous voltage value supplied to the electric load, compares the detected value with a target value, and, depending on the difference between the detected value and the target value, modifies the pulse width of the PWM signal. The instantaneous voltage value is thus forced to reach to the target value.
In the image forming apparatus, when, for example, a sheet enters between a transfer roller and a photoconductor, impedance between the transfer roller and the photoconductor drastically varies, which causes wide variation in the instantaneous voltage value (a transfer voltage value). The known power supply device, however, is configured to always wait for a constant time after modification of the pulse width of the PWM signal, and only after then, modify the next pulse width. Therefore, when impedance widely fluctuates as above, the known power supply device has a problem of follow-up delay in output from the high voltage generation device against the modification of the pulse width. A considered means for solving this problem is to shorten an execution time interval for modification of the pulse width (the above constant time), however, a shortened execution time interval may be affected by the follow-up delay and cause instability in the instantaneous transfer voltage value, which results in lower accuracy in power supply control. For example, if the transfer voltage is instable when transferring developer images on the photoconductor to the recording medium, transfer trouble and the like can occur, therefore resulting in lower transfer quality.